1. Field of the Invention
The present invention relates to a Multi-Input Multi-Output (MIMO) system, and more particularly, to a method for efficiently retransmitting signals in a MIMO system that uses a Hybrid Automatic Repeat reQuest (HARQ) scheme.
2. Discussion of the Related Art
Error control algorithms used for current communication systems can be mainly divided into two types, i.e., Automatic Repeat Request (ARQ) and Forward Error Correction (FEC). The ARQ type is divided into a stop and wait ARQ scheme, a go-back-N ARQ scheme, a selective-repeat ARQ scheme, etc. In the stop and wait ARQ scheme, the next frame is transmitted after correct receipt of each transmission (Tx) frame is confirmed through an acknowledgement (ACK) signal. In the Go-Back-N ARQ scheme, if it is determined that transmission is unsuccessful after N consecutive data frames are transmitted, then all data frames transmitted subsequent to an erroneous frame are retransmitted. In the Selective-Repeat ARQ scheme, only erroneous frames are selectively retransmitted.
On the other hand, the Hybrid Automatic Repeat request (HARQ) scheme combines the ARQ scheme and the FEC scheme to control errors and maximize the capability of an error correction code of received data when retransmission is performed. The HARQ scheme is mainly classified into a chase combining (CC) HARQ method and an incremental redundancy (IR) HARQ method according to the characteristics of bits transmitted when retransmission is performed. The CC HARQ method uses the same data as that used for initial transmission when retransmission is performed and increases a signal-to-noise ratio (SNR) at the receiving end, thereby acquiring a gain. The IR HARQ method transmits and combines redundancy bits when retransmission is performed so that the receiving end obtains a coding gain, thereby increasing performance.
The HARQ transmission method can be classified into a synchronous HARQ method and an asynchronous HARQ method. In the synchronous HARQ method, the transmitting end transmits data through predetermined resources at a specific time known to both the transmitting end and the receiving end. Therefore, in the HARQ transmission method, threr is no need to provide signaling information required for transmission, for example, an HARQ process number indicating the identity of data.
On the other hand, in the asynchronous HARQ method, data is transmitted using resources that are allocated at an arbitrary time. Therefore, in the asynchronous HARQ method, it is necessary to provide such signaling information, for example, an HARQ process number, which may increase signaling overhead.
FIG. 1 illustrates an example control signal structure for a conventional synchronous or asynchronous HARQ system.
Specifically, FIG. 1 shows an example control signal structure for use in the 3GPP communication system (See 3GPP TR 25.814). According to this communication standard, a control signal of the HARQ system can be transmitted using only 2-bit control information without the need to indicate the index of a currently transmitted process block. On the other hand, a control signal of the asynchronous HARQ system requires information of a greater number of bits including information required to indicate the index of a currently transmitted process block.
The control information structure shown in FIG. 1 can be represented by the following Table 1.
TABLE 1FieldSizeCommentAsynchronousHARQ process3HARQ process indicated byHARQnumbercurrent transmissionRedundancy2IR supportversionNew data1Soft buffer clearingindicatorSynchronousRetransmission2Used to derive redundancyHARQSequence Numberversion (for IR support)and new data indicator(for soft buffer clearing)
As can be seen from Table 1, up to 8 combinations of process block indices can be represented through a 3-bit control signal in the asynchronous HARQ system. Table 1 illustrates a control signal for use when the number of process blocks that can be transmitted for each transmission unit is 1. However, the number of transmittable process blocks for each transmission unit in the asynchronous HARQ system may increase in various ways, for example due to the number of process blocks that can be simultaneously transmitted. This may increase the number of bits of a control signal indicating the HARQ process number described above, thereby increasing system overhead.
In the following description, reference will be made to a variety of cases where the number of process blocks transmitted for each transmission unit increases and a description will also be given of a method for reasonably reducing the number of corresponding process block combinations in such cases.
FIG. 2 illustrates a stop-and-wait HARQ scheme.
As described above, the HARQ scheme combines the ARQ scheme and the FEC scheme to control errors and maximizes the capability of an error correction code of received data when retransmission is performed. Specifically, the receiving end (Rx) transmits an ACK signal if no error is detected in received data and transmits a NACK signal if an error is detected. When the transmitting end (Tx) receives an ACK signal, the transmitting end (Tx) transmits next data. When the transmitting end (Tx) receives a NACK signal, the transmitting end (Tx) retransmits the same data as that in which an error has occurred. Here, the format of retransmitted data may be changed according to the HARQ type.
Particularly, the stop-and-wait protocol of FIG. 2 delays transmission of data by a round trip time (RTT) that passes until the transmitting end (Tx) receives an ACK/NACK signal from the receiving end (Rx) and then transmits the delayed data. Although the stop-and-wait protocol of FIG. 2 is the simplest and most effective transmission method, the method may reduce link transmission efficiency due to delay of the RTT.
The following N-channel stop-and-wait HARQ scheme may be used to solve this problem.
FIG. 3 illustrates an N-channel stop-and-wait HARQ structure.
In the stop-and-wait HARQ scheme, a data receiving end can generally determine whether or not data has been successfully received using an error detection code such as a cyclic redundancy check (CRC) code. In the following description, each data unit in which an error can be detected will be referred to as an “HARQ process block” or simply referred to as a “process block” unless such use causes confusion, for ease of explanation. Identifiers used to discriminate between HARQ process blocks that can be transmitted within a predetermined interval, for example 1 Round Trip Time (RTT), in the system will be referred to as “HARQ process indices”.
In the general stop-and-wait HARQ scheme illustrated in FIG. 2, transmission of data is delayed by an RTT that passes until an ACK/NACK of a process block is received after the process block is transmitted as described above. On the other hand, in the N-channel stop-and-wait HARQ scheme illustrated in FIG. 3, N process blocks that can be transmitted within an RTT are transmitted and, thereafter, individual ACK/NACK signals for the N process blocks are received. While this method increases link efficiency, it may increase the number of process block indices that can be transmitted within an RTT by N times.
In the case where the system has a wide bandwidth for transmission or data is transmitted using a MIMO scheme, a number of HARQ process blocks can be simultaneously transmitted.
FIG. 4 illustrates a transmission method based on a multiple HARQ processing scheme.
As shown in FIG. 4, m HARQ processes can simultaneously transmit m HARQ process blocks at a specific time. When the receiving end has received data, it can transmit m ACK/NACK signals for m HARQ process blocks to the transmitting end. The method in which m process blocks are simultaneously transmitted in this manner can be combined with the N-channel stop-and-wait method as shown in FIG. 3 to increase system link performance.
In the case where a plurality of process blocks is simultaneously transmitted in this manner, each HARQ process through which each process block is transmitted is referred to as a “layer” for ease of explanation in the following description. The layer may correspond to either each band when a plurality of process blocks is simultaneously transmitted due to a wide system bandwidth in a communication system or each antenna through which data is simultaneously transmitted in a MIMO communication system. The following is a brief description of the MIMO communication system as an example system which employs a plurality of layers as described above.
In the Multi-Input Multi-Output (MIMO) scheme, a base station and a mobile terminal each include two or more antennas to transmit data through multiple paths and the receiving end detects signals received through the paths. This MIMO scheme can be classified into a variety of schemes such as spatial diversity, transmit diversity, beamforming, spatial multiplexing for a single user, spatial multiplexing for multiple users, etc.
In the spatial diversity scheme, the same data is transmitted through a plurality of antennas. This scheme enables reliable operation when the reliability of Channel Quality Information (CQI) feedback from a terminal is low due to fading. In addition, in the case where there is a need to provide a service containing traffic sensitive to delay, the spatial diversity scheme can efficiently cope with the fading using diversity without waiting for a good channel condition. As a representative MIMO communication technology, the transmit diversity scheme can be used when the transmitter has multiple antennas and a channel condition is unknown.
On the other hand, the beamforming scheme assigns different weights according to channel conditions to signals of multiple antennas to increase a Signal to Interference plus Noise Ratio (SINR). In the case of the transmit beamforming scheme, it is necessary to provide an additional feedback since there are difficulties in determining channel conditions and thus how to efficiently support the feedback is an important factor in system design.
The following is a brief description of both the spatial multiplexing scheme for a single user and the spatial multiplexing scheme for multiple users.
FIG. 5 illustrates a Spatial Multiplexing (SM) scheme and a Spatial Division Multiple Access (SDMA) scheme for use in a MIMO communication system.
The spatial multiplexing scheme for a single user is also referred to as SM or Single User MIMO (SU-MIMO). In this scheme, data is transmitted through multiple antennas of one user in the manner as shown in the left side of FIG. 5. Thus, the MIMO-channel capacity increases in proportion to the number of antennas. On the other hand, the spatial multiplexing scheme for multiple users is referred to as SDMA or Multi-User MIMO (MU-MIMO). In this scheme, data is transmitted and received through antennas of multiple users in the manner as shown in the right side of FIG. 5.
When data is transmitted in the MIMO mode, it is necessary to add a variety of information such as a rank index, a Precoding Vector Index (PVI), and an interference vector. The rank index is an index used to indicate the number of transmission ranks assigned to each allocated resource element and the PVI indicates a preceding vector which a UE (or terminal) will use in each resource element allocated in a resource allocation field.
On the other hand, when the MIMO scheme described above is applied in two modes, i.e., a Single CodeWord (SCW) mode and a Multi-CodeWord (MCW) mode. In the SCW mode, a single codeword, which is an error-detectable unit, is simultaneously transmitted through multiple antennas. In the MCW mode, several codewords are simultaneously transmitted through multiple antennas.
FIG. 6 illustrates a structure of a transmitting end in a multiple codeword (MCW) MIMO system.
As shown in FIG. 6, encoding (e.g., turbo-encoding of FIG. 6) and modulation (e.g., QAM modulation of FIG. 6) are performed on M data packets to produce M codeword HARQ process blocks. The M codeword HARQ process blocks are mapped to layers at the MIMO portion and the layers are then combined with an efficient antenna signaling according to the number of (Mt) physical antennas and are then transmitted to the receiving end. Thereafter, the receiving end feeds back channel quality information of each antenna so that the coding rate and the modulation scheme can be adjusted according to the channel quality information.
On the other hand, codewords and physical antennas may have mapping relationships described below.
FIG. 7 illustrates an example mapping relationship between codewords and physical antennas.
Specifically, FIG. 7 illustrates codeword-to-layer mapping for spatial multiplexing in downlink in 3GPP TS 36.211.
As illustrated in FIG. 7, in the case of rank 1, one codeword is mapped to one layer and is then transmitted through four antennas via a precoder. In the case of rank 2, two codewords are mapped to two layers and the two layers are then mapped to four antennas through a precoder. In the case of rank 3, one of two codewords is mapped to two layers through a serial-to-parallel (S/P) converter such that a total of two codewords are mapped to three layers and the three layers are then mapped to four antennas through a precoder. In the case of rank 4, each of two codewords is mapped to two layers through an S/P converter such that a total of four layers are mapped to four antennas through a precoder. The number of simultaneously transmitted codewords (i.e., HARQ process blocks) can be determined based on the number of ranks.
However, in the case where one codeword is retransmitted among a plurality of transmitted codewords due to failure of transmission of the codeword after data is transmitted using the rank 3 or the rank 4 in the system which uses the codeword-to-layer mapping schemes described above with reference to FIG. 7, the rank number for transmission should be forcibly reduced from the rank 3 or the rank 4 shown in FIG. 7 to retransmit the codeword. In this case, it is difficult to use the HARQ chase combining described above and only half of the resources used for initial transmission are used, thereby reducing efficiency. In addition, if a high coding rate is applied to transmit data when initial transmission is performed, the coding gain may be significantly reduced when retransmission is performed.
Accordingly, there is a need to provide an efficient data processing method for retransmitting signals from a transmitting end in a MIMO system that uses an HARQ scheme in order to overcome the above problems.